sinopec is buying the stake from chesapeake energy, which has been looking to sell off assets in order to raise money and pay down debt. the chinese company is spending just over $1 billion. but that price was lower than expectations, sending shares of chesapeake tumbling 6.8%. volume was heavy with the stock finishing at a three week low. this deal markets just the latest buyout of u.s. energy fields by a foreign company. the biggest deal of the day was in health care with irish drug maker elan the target of an offer from a pharmaceutical investment firm. royalty pharma said it approached elan last week about a take over. after talks, the offer was $6.6 billion, or $11 per share. the two companies reportedly have not had any talks since. shares of elan rallied 5.1%, closing above the offer price, indicating the market thinks a higher price could be coming. just this month elan announced a $3.3 billion deal to sell its share of multiple sclerosis drug tysabri to its partner. the drug accounts for most of elan's revenue. bio-pharmaceutical company affymax was a half billion dollar firm